In modern surgery, one of the most important instruments available to medical personnel is the powered surgical drill. Typically, this drill comprises a housing in which a motor is secured. The motor has a shaft that is connected to some type of chuck or other coupling assembly that is mounted to the housing. The coupling assembly holds a cutting accessory that is applied to the patient in order to perform a specific medical procedure. Some common cutting accessories are drill bits, burs and reamers. These accessories are used to drill into and/or separate sections of soft tissue and hard tissue, commonly referred to as bone. The ability to use surgical drills to actuate these and other cutting accessories has lessened the physical strain of physicians and other medical personnel that perform these medical procedures. Moreover, most surgical procedures can be performed more quickly and more accurately with powered surgical tools than with the manual equivalents that preceded them.
It is often desirable to increase the rotation force, the torque, developed by the motor internal to a surgical drill. Many drills are provided with internal gear assemblies that perform this function. Since an inevitable result of a gear assembly's increasing torque output is a decrease in rotational speed, these gear assemblies are referred to as speed reduction gear assemblies. Generally, the input-to-output speed reduction factor and the input-to-output torque amplification factor of these gear assemblies are identical. Moreover, manufacturers of surgical drills often provide removable drill attachments for coupling to drills that have their own speed reduction gear assemblies. The ability to selectively couple a drill attachment to a surgical tool makes it possible for a surgeon to even further increase the torque available to the cutting tool coupled to the drill. Often, these drill attachments are designed to reduce the speed/increase the torque by a pre-set whole number ratio. For example, drill attachments with internal gear assemblies that decrease speed of the motor drive shaft by 3:1 or 4:1 have been provided. (It should be understood that the above ratio refers to the relationship of the input shaft speed to the output shaft speed. The reciprocal of these ratios give the relationship between torque input and torque output.)
Often, planetary gear assemblies are used as speed reduction gear assemblies for both surgical drills and the removable drill attachments designed for use with these drills. A planetary gear assembly has a centrally located sun gear that is directly coupled to the drill drive shaft that provides the basic motive power. A set of gears, referred to as planet gears, are located around and engage the sun gear. A fixed ring surrounds the planetary gears. The inner surface of the ring has teeth which the planetary gears engage. The planet gears are connected to a common planet carrier. The planet carrier has a number of arms to which the individual planet gears are attached. When the sun gear is rotated, the sun gear transfers the rotational energy it receives to the planet gears. The planet gears, in turn, rotate both around their own axes and the axis of the sun gear. The rotation of the planet gears about the sun gear causes a like rotation of the planet carrier.
Planetary gear assemblies and other gear assemblies have proven to be useful assemblies for reducing the speed/increasing the torque of the shafts connected to surgical drills. However, to date, it has been difficult to provide a removable drill attachment with a planetary gear assembly or other gear assembly that can be used to achieve a 2:1 speed reduction. This is because physically, owing to the relationships between the components of a conventional planetary gear assembly, it is impossible to dimension the components so that they cause an exact 2:1 speed reduction. The only way it is possible to form a conventional planetary gear assembly that achieves approximately a 2:1 speed reduction is to provide it with very small planet gears. Such an assembly would not be able to function for a long time at the high rotational speeds at which most surgical drills operate. Attempts to provide other gear assemblies that reduce rotational speed/increase torque by a factor of 2 have proven unsuccessful for use with surgical drills. While these other gear assembles provide the desired speed reduction/increased torque, the direction of rotation of their output shafts is reversed from the direction input shaft rotation. It is impractical to provide removable drill attachments chucks with these gear assemblies since the change in rotation of the associated cutting accessory could be confusing to a surgeon in the middle of a surgical procedure.